1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example, if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
In interferometry, it is possible to accurately determine displacements by measuring an interference pattern. Thereto, use may be made of beams of radiation that are made to overlap each other. A component that is used in many interferometry arrangements is a retroreflecting device, or retroreflector. This item is useful, in that it is able to reflect a beam of radiation back into the same direction of incidence, not just for one particular direction, as is any plane mirror, but for a whole range of directions. This retroreflection property may be achieved using a retroreflector surface, which generally consists of three mutually perpendicular faces, such as of a cube. A retroreflector is also called a corner cube, because the three mentioned faces may be considered a corner of a cube. Furthermore, a retroreflector has a surface through which a beam of radiation may enter the retroreflector, and may leave again after being retroreflected.
Conventionally, a retroreflector is arranged in combination with a beam splitter device, which is configured to split up a single beam of radiation into two (or more) beams of radiation. The retroreflector, as well as one or more mirrors, may be used to achieve overlap of the two (or more) beams of radiation. Thereby, an interference pattern may be obtained. Changes of the interference pattern may be used to determine a displacement.
For interferometry purposes, it is desirable that the quality of the faces and surfaces of the retroreflector be excellent. For example, it is not uncommon for the flatness of the faces to be better than 0.1 wavelength of the radiation used, e.g. better than 50 nm. Furthermore, it is desirable that the (local) surface roughness be no more than a few nanometers. In order to handle the retroreflector, without touching the functional faces with the extremely high quality surface, most if not all retroreflectors have a handler surface. This handler surface may be a surrounding portion configured to handle the retroreflector, which is sometimes called a handler portion in this context. If retroreflectors do not have such a handler portion, functional surfaces, such as the retroreflector surface, may be damaged during operation. This may occur when, for example, the retroreflector is being handled, e.g. mounted in a beam splitter or other optical device.
U.S. Pat. No. 4,504,147 discloses an alignment sensor with a corner cube that has a rounded, cylindrical handler surface adjacent the three retroreflector faces. The corner cube is coupled to a beam splitter, with an optical cement.
Similarly, a tetrahedron, which is no more than a cut off corner of a cube, generally requires such a handler portion. Such a tetrahedron has a triangular base face. Many retroreflectors have been given a rectangular base face by cutting and truncating the faces of the tetrahedron. This automatically provides a handler surface by means of the additional (truncated) side faces.
Such a handler surface or handler portion may give rise to various disadvantages. First of all, the handler portion relates to an amount of material, which is only used when handling the retroreflector. After mounting, the handler portion is useless, which is a waste of material. Furthermore, the mounting of a retroreflector in e.g. an interferometry system with a beam splitter etc. is highly critical, and requires a high accuracy and stability in time. The various known ways of mounting a retroreflector are not always satisfactory in this respect.